1. Field of the Invention
This invention relates to hydraulic turbines and, more particularly, to low head open flume hydraulic turbine installations. Additionally, this application relates to open flume hydraulic turbine installations having provisions for vortex suppression.
2. Description of the Prior Art
Intakes for hydraulic turbine installations come in a variety of designs For example, the list of prior art intakes includes penstocks, siphon intakes, open flumes and closed flumes. Each of these prior art methods for providing an intake for a hydraulic turbine installation differ from each other and include various design constraints.
The present invention relates to an open flume installation for a low head hydraulic turbine. An open flume installation typically comprises a flume defined by a back wall, a floor and a pair of laterally spaced vertical side walls. The side walls, floor and back wall define an open flume in communication with an upper elevation water source (such as a river or reservoir). Open flume installations are distinguishable from closed flume installations since the closed flume installation has a higher head loss due to a high velocity of water flow which results in higher friction against the ceiling of the closed flume. The higher velocity through a closed flume relocates vortex formation problems to the intake to the closed flume.
Open flume installations are old in the art and include vertical axis and horizontal axis turbine installation. In vertical axis open flume installations, a water passageway was formed in the installation support structure with an intake provided at the floor of the flume and an outlet provided submerged within a low elevation chamber or tailwater. A turbine runner was provided within the intake for rotation about a vertical axis. A vertical shaft projected from the runner upwardly to a generator. An example of such a vertical axis open flume installation is shown in the enclosed Allis-Chalmers Bulletin No. 54B10241-03 on page 7. A horizontal axis turbine in an open flume installation would conventionally have a similar flume width.
As is known in the art, open flume installations with either horizontal or vertical axis turbines are subject to certain dimension restrictions. For example, in a vertical axis turbine open flume installation, the width of the flume (the distance between the side walls) is three times the diameter of the turbine runner. Likewise, the depth of the elbow-shaped draft tube is about 2.7 times the diameter of the runner. Also, the center line of the runner axis is displaced from the back wall a distance of approximately 1.4 times the runner diameter. In horizontal axis open flume installations, similar dimension restrictions apply. These and other restrictions are enumerated in the above referenced bulletin.
The above referenced dimension restrictions found in vertical axis and horizontal axis open flume turbine installations are due to the flow pattern of water within the flume approaching the turbine intake. The dimension restrictions are necessary to prevent formation of vortices at the turbine intake. When a vortex occurs, air is included within the water flowing through the turbine which decreases the efficiency of the turbine and can cause unstable operation. Vortex formation is abated by following the above described dimension restrictions. Likewise, vortex formation can be abated by increasing the depth of the turbine intake. When vortex formation does occur at turbine installations, attempts are made to break up the vortex by floating logs or rafts in the area of vortex formation.
While the above referenced dimension restrictions may aid in abating vortex formation, they may result in construction costs for a particular hydraulic installation being so high as to make the installation uneconomical. In addition, for a given size open flume, the size of the hydraulic turbine is limited by the above installation restrictions. Conversely, for a given size turbine runner design, adequate excavation and civil engineering work must be incurred to provide a sufficiently large flume to account for the above described typical minimum dimension restrictions. Likewise, in the case of the vertical axis turbine, significant excavation is required to form the draft tube.